我已经坚持了几天,并且可以真正使用一些帮助和了解如何使用 DiffUtil 设置 RecylerView(RV),似乎无论我尝试什么我都无法更新 RV。我将介绍我所拥有的,并希望有人可以阐明我做错了什么。
应用启动
- 创建视图模型,
- 然后将 LiveDataList 填充到 ViewModel 中。
- RV 已实例化
- LayoutManager 和 ListAdapter 应用于 RV。
- ItemTouchHelper 附加到 RecyclerView 允许在向左滑动时从 LiveDataList 中删除项目。
- 观察LiveDataList,我的理解是当列表发生变化时,观察者会将列表提交给AdapterList,如果原来提交的列表和修改后的列表有差异,AdapterList会更新回收站视图。
为什么从列表 LiveDataList 中添加或删除项目时不调用观察者进行更新。
class MainActivity : AppCompatActivity() {
var myAdapter = RVAdapter()
private lateinit var viewModel: MyViewModel()
override fun onCreateView(savedInstanceState: Bundle? ) {
super.onCreate(savedInstanceState)
// Initialize the ViewModel, that stores the userList.
viewModel = ViewModelProvider(this).get(MyViewModel::class.java)
// Creates list of users
addUser()
// Link an RV object to view
val rv = findViewById<RecyclerView>(R.id.recycler_view)
// apply RV Settings.
rv.apply {
layoutManager = LinearLayoutManager(baseContext, LinearLayoutManager.VERTICAL, false)
adapter= myAdapter
}
ItemTouchHelper(object : ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.LEFT) {
override fun onMove(...) {
// move items isn't used.
}
override fun onSwipe(viewHolder: ViewHolder, direction: Int) {
// When I swipe a second item, the app crashes.
val editList: MutableList<User>? = viewModel.usersList?.value as MutableList<User>?
editList?.removeAt(viewHolder.adapterPosition)
viewModel.usersList?.postValue(userList)
}
}).attachtoRecyclerView(rv)
// The observer will run after swiping an item but the UI doesn't update.
viewModel.usersList?observe(this, Observer {
it?.let {
// Should update list but it doesn't
myAdapter.submitList(it)
}
}
private fun addUser() {
// Shouldn't there be away to add Items directly to list in ViewModel.
val newList: MutableList<User> = mutableListOf()
newList.add(User("Shawn", 1)
newList.add(User("Shannon", 2)
newList.add(User("Don", 3)
viewModel.userList?.postValue(newList)
}
数据类:这是非常基本的,在这里发生的事情并不多。
data class User(val name: String, val accountNumber: Int) {
}
ViewModel:MyViewModel 只存储 MutableLiveDat
class MyViewModel : ViewModel() {
val usersList: MutableLiveData<List<User>>? = MutableLiveData()
}
RecycleView 适配器
class RVAdapter : ListAdapter<User, RVAdapter.ViewHolder>(MyDiffCallback() {
// This DiffUtil class never gets called not even on startup.
class MyDiffCallback: DiffUtil.ItemCallback<User>() {
override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {
return oldItem.name == newItem.name
}
override fun areContentsTheSame(oldItem: User, newItem: User): Boolean {
return oldItem == newItem
}
}
... Rest of RV is working fine but can be included if it's helpful.
}
当从 LiveData 列表中删除或添加项目时,观察者会将修改后的列表传递给 ListAdapter,可以做什么?
StackTrace 包括索引出站。
2020-10-05 20:14:16.538 19042-19042/com.example.practicerecyclerview2 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.practicerecyclerview2, PID: 19042
java.lang.IndexOutOfBoundsException: Index: 2, Size: 2
at java.util.ArrayList.remove(ArrayList.java:503)
at com.example.practicerecyclerview2.MainActivity$onCreate$2.onSwiped(MainActivity.kt:100)
at androidx.recyclerview.widget.ItemTouchHelper$4.run(ItemTouchHelper.java:712)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)